Porcine proliferative enteropathy (PPE or PE) has become an important disease of the modern pig industry world-wide. The disease affects 15% to 50% of the growing herds and up to 30% of the individual animals in established problem herds. Today annual economical losses have been estimated US$ 5-10 in extra feed and facility time costs per affected pig. PPE is a group of chronic and acute conditions of widely differing clinical signs (death, pale and anaemic animals, watery, dark or bright red diarrhoea, depression, reduced appetite and reluctance to move, retarded growth and increased FCR). However there are two consistent features. The first, a pathological change only visible at necropsy, is a thickening of the small intestine and colon mucosa. The second is the occurrence of intracytoplasmatic small-curved bacteria in the enterocytes of the affected intestine. These bacteria have now been established as the etiological agent of PPE and have been name Lawsonia intracellularis. 
Over the years Lawsonia intracellularis has been found to affect virtually all animals including monkeys, rabbits, ferrets, hamsters, fox, horses, and other animals as diverse as ostrich and emoe. Lawsonia intracellularis is a gram-negative, flagellated bacterium that multiplies in-eukaryotic enterocytes only and no cell-free culture has been described. In order to persist and multiply in the cell Lawsonia intracellularis must penetrate dividing crypt cells. The bacterium associates with the cell membrane and quickly enters the enterocyte via an entry vacuole. This then rapidly breaks down (within 3 hours) and the bacteria flourish and multiply freely in the cytoplasm. The mechanisms by which the bacteria cause infected cells to fail to mature, continue to undergo mitosis and form hypoplastic crypt cells is not yet understood.
The current understanding of Lawsonia intracellularis infection, treatment and control of the disease has been hampered by the fact that Lawsonia intracellularis can not be cultivated in cell-free media. Although there are reports of successful co-culturing Lawsonia intracellularis in rat enterocytes this has not lead to the development of vaccines for combating Lawsonia intracellularis, although there clearly is a need for such vaccines.